Polyimide films and electronic devices

ABSTRACT

In a first aspect, a polyimide film includes a dianhydride and a diamine. The dianhydride, the diamine or both the dianhydride and the diamine include an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer. The polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm. In a second aspect, an electronic device includes the polyimide film of the first aspect.

FIELD OF DISCLOSURE

The field of this disclosure is polyimide films and electronic devices.

BACKGROUND OF THE DISCLOSURE

Polyimide films can potentially replace rigid glass cover sheets and other substrates which are currently used in display applications, such as organic light-emitting diode (OLED) displays. For example, aromatic polyimides are typically very thermally stable, with glass transition temperatures (T_(g)) of greater than 320° C., and have excellent foldability and rollability, a critical property needed for next-generation flexible displays. For polyimide films used in display applications, in addition to having high transmittance and low haze, the polyimide film also needs to be neutral in color. Typical specifications require that both a* and b* are no greater than 1 color unit from neutral (0) in CIE L*, a*, b* color space coordinates, i.e., the absolute values of a* and b* should be less than 1. The three coordinates of CIE L*, a*, b* represent: (1) the lightness of the color (L*=0 yields black and L*=100 indicates diffuse white), (2) its position between red/magenta and green (negative a* values indicate green, while positive values indicate magenta) and (3) its position between yellow and blue (negative b* values indicate blue and positive values indicate yellow).

Typical polyimides with fluorinated monomers, which are nearly colorless, still absorb light in the blue or violet wavelengths (400-450 nm) which gives the films a yellow appearance in transmission. The color of the polyimide films is primarily generated from charge transfer absorptions arising from HOMO-LUMO transitions which can occur both within the polymer chains and between polymer chains. Various approaches have been used to alter HOMO-LUMO transition energies or to frustrate interchain interactions. In one approach, a fluorinated monomer is used to alter the HOMO-LUMO transition energies of the polyimide polymer, but still some residual yellow color can be apparent in these polyimide films. Depending on the monomer composition in the polyimide, therefore, b* can be higher than 1. Since the CIE L*, a*, b* color measurement of a film is also dependent on its thickness, achieving a neutral appearance is even more difficult for thicker films, such as those greater than 25 μm.

In addition to having good optical properties, polyimide films used in these applications need to maintain good mechanical properties, such as a high elastic modulus. The modulus of a polyimide film can be increased by incorporating more rigid monomers into the polyimide backbone. In the case of rigid aromatic monomers, however, charge transfer absorptions, as described above, lead to higher color for a polyimide incorporating these monomers. Additionally, for rigid non-aromatic monomers, their poor thermal stability at higher temperatures, such as typical imidization temperatures, may lead to decomposition of the monomer, resulting in increased color. These are just two examples of how using more rigid monomers, while improving mechanical properties, may increase the color of polyimides.

Alicyclic and aliphatic monomers, when incorporated into a polyimide structure, can lower color by modifying the electronic structure and charge transfer characteristics of the polymer. These monomers would not, by themselves, contribute to any charge transfer transitions. However, a process where the film is formed by casting a polyamic acid solution and curing of the film which is produced results in significant color. The generation of color is more pronounced when curing is performed in air, indicating that a secondary color formation mechanism is occurring.

SUMMARY

In a first aspect, a polyimide film includes a dianhydride and a diamine. The dianhydride, the diamine or both the dianhydride and the diamine include an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer. The polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm.

In a second aspect, an electronic device includes the polyimide film of the first aspect.

In a third aspect, a polyimide film includes a dianhydride and a diamine. The dianhydride, the diamine or both the dianhydride and the diamine include an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer. The polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm. The polyimide film is formed by:

(a) polymerizing the dianhydride and the diamine in the presence of a first solvent to obtain a polyamic acid solution;

(b) imidizing the polyamic acid solution to form a substantially imidized solution;

(c) casting the substantially imidized solution to form a film; and

(d) drying the film.

In a fourth aspect, a polyimide film includes a dianhydride and a diamine. The dianhydride, the diamine or both the dianhydride and the diamine include an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer. The polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm. The polyimide film is formed by:

(a) polymerizing the dianhydride and the diamine in the presence of a first solvent to obtain a polyamic acid solution;

(b) imidizing the polyamic acid solution to form a first substantially imidized solution;

(c) precipitating a solid polyimide resin from the first substantially imidized solution with an antisolvent;

(d) isolating and drying the solid polyimide resin;

(e) dissolving the solid polyimide resin in a second solvent to form a second substantially imidized solution;

(f) casting the second substantially imidized solution to form a film; and

(g) drying the film.

The foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as defined in the appended claims.

DETAILED DESCRIPTION

In a first aspect, a polyimide film includes a dianhydride and a diamine. The dianhydride, the diamine or both the dianhydride and the diamine include an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer. The polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm.

In one embodiment of the first aspect, the polyimide film further includes a sub-micron filler. In a specific embodiment, the sub-micron filler is selected from the group consisting of silicon oxide, titanium oxide, aluminum oxide, zirconium oxide, and mixtures thereof. In another specific embodiment, the sub-micron filler has a size of less than 100 nm in at least one dimension.

In another embodiment of the first aspect, the dianhydride is selected form the group consisting of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride and mixtures thereof. In a specific embodiment, the dianhydride further includes an alicyclic dianhydride selected from the group consisting of cyclobutane-1,2,3,4-tetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, hexahydro-4,8-ethano-1H,3H-benzo[1,2-c:4,5-c′]difuran-1,3,5,7-tetrone, 3-(carboxymethyl)-1,2,4-cyclopentanetricarboxylic 1,4:2,3-dianhydride and meso-butane-1,2,3,4-tetracarboxylic dianhydride.

In still another embodiment of the first aspect, the diamine includes a fluorinated aromatic diamine. In a specific embodiment, the fluorinated aromatic diamine comprises 2,2′-bis(trifluoromethyl) benzidine.

In yet another embodiment of the first aspect, the diamine includes an aliphatic diamine selected from the group consisting of 1,2-diaminoethane, 1,6-diaminohexane, 1,4-diaminobutane, 1,5 diaminopentane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,9-diaminononane, 1,10-diaminodecane, 1,11-diaminoundecane, 1,12-diaminododecane, 1,16-hexadecamethylenediamine, 1,3-bis(3-aminopropyl)-tetramethyldisiloxane, isophoronediamine, bicyclo[2.2.2]octane-1,4-diamine and mixtures thereof.

In still yet another embodiment of the first aspect, the diamine includes an alicyclic diamine selected from the group consisting of cis-1,3-diaminocyclobutane trans-1,3-diaminocyclobutane, 6-amino-3-azaspiro[3.3]heptane, and 3,6-diaminospiro[3.3]heptane, bicyclo[2.2.1]heptane-1,4-diamine, isophoronediamine, bicyclo[2.2.2]octane-1,4 diamine, cis-1,4 cyclohexanediamine, trans-1,4 cyclohexanediamine, 1,4-bis(aminomethyl)cyclohexane, 4,4′-methylenebis(cyclohexylamine), 4,4′-methylenebis(2-methyl-cyclohexylamine), bis(aminomethyl)norbornane and mixtures thereof.

In a further embodiment of the first aspect, the polyimide film has a thickness in the range of from 10 to 150 μm.

In still a further embodiment of the first aspect, the polyimide film has a L* of at least 90.

In still yet a further embodiment of the first aspect, the polyimide film has a haze of less than 1%.

In a second aspect, an electronic device includes the polyimide film of the first aspect.

In one embodiment of the second aspect, the polyimide film is used in device components selected from the group consisting of substrates for color filter sheets, cover sheets, and touch sensor panels.

In a third aspect, a polyimide film includes a dianhydride and a diamine. The dianhydride, the diamine or both the dianhydride and the diamine include an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer. The polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm. The polyimide film is formed by:

(a) polymerizing the dianhydride and the diamine in the presence of a first solvent to obtain a polyamic acid solution;

(b) imidizing the polyamic acid solution to form a substantially imidized solution;

(c) casting the substantially imidized solution to form a film; and

(d) drying the film.

In one embodiment of the third aspect, after (b) and before (c), the substantially imidized solution is filtered to remove insoluble constituents of the solution.

In a fourth aspect, a polyimide film includes a dianhydride and a diamine. The dianhydride, the diamine or both the dianhydride and the diamine include an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer. The polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm. The polyimide film is formed by:

(a) polymerizing the dianhydride and the diamine in the presence of a first solvent to obtain a polyamic acid solution;

(b) imidizing the polyamic acid solution to form a first substantially imidized solution;

(c) precipitating a solid polyimide resin from the first substantially imidized solution with an antisolvent;

(d) isolating and drying the solid polyimide resin;

(e) dissolving the solid polyimide resin in a second solvent to form a second substantially imidized solution;

(f) casting the second substantially imidized solution to form a film; and

(g) drying the film.

In one embodiment of the fourth aspect, after (e) and before (f), the second substantially imidized solution is filtered to remove insoluble constituents of the solution.

In another embodiment of the fourth aspect, the first and second solvents are the same or different.

Many aspects and embodiments have been described above and are merely exemplary and not limiting. After reading this specification, skilled artisans appreciate that other aspects and embodiments are possible without departing from the scope of the invention. Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.

Depending upon context, “diamine” as used herein is intended to mean: (i) the unreacted form (i.e., a diamine monomer); (ii) a partially reacted form (i.e., the portion or portions of an oligomer or other polymer precursor derived from or otherwise attributable to diamine monomer) or (iii) a fully reacted form (the portion or portions of the polymer derived from or otherwise attributable to diamine monomer). The diamine can be functionalized with one or more moieties, depending upon the particular embodiment selected in the practice of the present invention.

Indeed, the term “diamine” is not intended to be limiting (or interpreted literally) as to the number of amine moieties in the diamine component. For example, (ii) and (iii) above include polymeric materials that may have two, one, or zero amine moieties. Alternatively, the diamine may be functionalized with additional amine moieties (in addition to the amine moieties at the ends of the monomer that react with dianhydride to propagate a polymeric chain). Such additional amine moieties could be used to crosslink the polymer or to provide other functionality to the polymer.

Similarly, the term “dianhydride” as used herein is intended to mean the component that reacts with (is complimentary to) the diamine and in combination is capable of reacting to form an intermediate (which can then be cured into a polymer). Depending upon context, “anhydride” as used herein can mean not only an anhydride moiety per se, but also a precursor to an anhydride moiety, such as: (i) a pair of carboxylic acid groups (which can be converted to anhydride by a de-watering or similar-type reaction); or (ii) an acid halide (e.g., chloride) ester functionality (or any other functionality presently known or developed in the future which is) capable of conversion to anhydride functionality.

Depending upon context, “dianhydride” can mean: (i) the unreacted form (i.e. a dianhydride monomer, whether the anhydride functionality is in a true anhydride form or a precursor anhydride form, as discussed in the prior above paragraph); (ii) a partially reacted form (i.e., the portion or portions of an oligomer or other partially reacted or precursor polymer composition reacted from or otherwise attributable to dianhydride monomer) or (iii) a fully reacted form (the portion or portions of the polymer derived from or otherwise attributable to dianhydride monomer).

The dianhydride can be functionalized with one or more moieties, depending upon the particular embodiment selected in the practice of the present invention. Indeed, the term “dianhydride” is not intended to be limiting (or interpreted literally) as to the number of anhydride moieties in the dianhydride component. For example, (i), (ii) and (iii) (in the paragraph above) include organic substances that may have two, one, or zero anhydride moieties, depending upon whether the anhydride is in a precursor state or a reacted state. Alternatively, the dianhydride component may be functionalized with additional anhydride type moieties (in addition to the anhydride moieties that react with diamine to provide a polymer). Such additional anhydride moieties could be used to crosslink the polymer or to provide other functionality to the polymer.

Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described herein.

When an amount, concentration, or other value or parameter is given as either a range, preferred range or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.

In describing certain polymers, it should be understood that sometimes applicants are referring to the polymers by the monomers used to make them or the amounts of the monomers used to make them. While such a description may not include the specific nomenclature used to describe the final polymer or may not contain product-by-process terminology, any such reference to monomers and amounts should be interpreted to mean that the polymer is made from those monomers or that amount of the monomers, and the corresponding polymers and compositions thereof.

The materials, methods, and examples herein are illustrative only and, except as specifically stated, are not intended to be limiting. As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a method, process, article, or apparatus that comprises a list of elements is not necessarily limited only those elements but may include other elements not expressly listed or inherent to such method, process, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

Also, use of the “a” or “an” are employed to describe elements and components of the invention. This is done merely for convenience and to give a general sense of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

Organic Solvents

Useful organic solvents for the synthesis of the polymers of the present invention are preferably capable of dissolving the polymer precursor materials. Such a solvent should also have a relatively low boiling point, such as below 225° C., so the polymer can be dried at moderate (i.e., more convenient and less costly) temperatures. A boiling point of less than 210, 205, 200, 195, 190, or 180° C. is preferred.

Useful organic solvents include: N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), methyl ethyl ketone (MEK), N,N′-dimethyl-formamide (DMF), dimethyl sulfoxide (DMSO), tetramethyl urea (TMU), glycol ethyl ether, diethyleneglycol diethyl ether, 1,2-dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), 1,2-bis-(2-methoxyethoxy) ethane (triglyme), gamma-butyrolactone, and bis-(2-methoxyethyl) ether, tetrahydrofuran (THF), ethyl acetate, hydroxyethyl acetate glycol monoacetate, acetone and mixtures thereof. In one embodiment, preferred solvents include N-methylpyrrolidone (NMP) and dimethylacetamide (DMAc).

Diamines

In one embodiment, a suitable diamine for forming the polyimide film can include an aliphatic diamine, such as 1,2-diaminoethane, 1,6-diaminohexane, 1,4-diaminobutane, 1,5 diaminopentane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,9-diaminononane, 1,10-diaminodecane (DMD), 1,11-diaminoundecane, 1,12-diaminododecane (DDD), 1,16-hexadecamethylenediamine, 1,3-bis(3-aminopropyl)-tetramethyldisiloxane, and combinations thereof. Other aliphatic diamines suitable for practicing the invention include those having six to twelve carbon atoms or a combination of longer chain and shorter chain diamines so long as both developability and flexibility are maintained. Long chain aliphatic diamines may increase flexibility.

In one embodiment, a suitable diamine for forming the polyimide film can include an alicyclic diamine (can be fully or partially saturated), such as a cyclobutane diamine (e.g., cis- and trans-1,3-diaminocyclobutane, 6-amino-3-azaspiro[3.3]heptane, and 3,6-diaminospiro[3.3]heptane), bicyclo[2.2.1]heptane-1,4-diamine, isophoronediamine, and bicyclo[2.2.2]octane-1,4 diamine. Other alicyclic diamines can include cis-1,4 cyclohexane diamine, trans-1,4 cyclohexane diamine, 1,4-bis(aminomethyl)cyclohexane, 4,4′-methylenebis(cyclohexylamine), 4,4′-methylenebis(2-methyl-cyclohexylamine), bis(aminomethyl)norbornane.

In one embodiment, a suitable diamine for forming the polyimide film can further include a fluorinated aromatic diamine, such as 2,2′-bis(trifluoromethyl) benzidine (TFMB), trifluoromethyl-2,4-diaminobenzene, trifluoromethyl-3,5-diaminobenzene, 2,2′-bis-(4-aminophenyl)-hexafluoro propane, 4,4′-diamino-2,2′-trifluoromethyl diphenyloxide, 3,3′-diamino-5,5′-trifluoromethyl diphenyloxide, 9.9′-bis(4-aminophenyl)fluorene, 4,4′-trifluoromethyl-2,2′-diaminobiphenyl, 4,4′-oxy-bis-[2-trifluoromethyl)benzene amine] (1,2,4-OBABTF), 4,4′-oxy-bis-[3-trifluoromethyl)benzene amine], 4,4′-thio-bis-[(2-trifluoromethyl)benzene-amine], 4,4′-thiobis[(3-trifluoromethyl)benzene amine], 4,4′-sulfoxyl-bis-[(2-trifluoromethyl)benzene amine, 4,4′-sulfoxyl-bis-[(3-trifluoromethyl)benzene amine], 4,4′-keto-bis-[(2-trifluoromethyl)benzene amine], 1,1-bis[4′-(4″-amino-2″-trifluoromethylphenoxy)phenyl]cyclopentane, 1,1-bis[4′-(4″-amino-2″-trifluoromethylphenoxy)phenyl]cyclohexane, 2-trifluoromethyl-4,4′-diaminodiphenyl ether; 1,4-(2′-trifluoromethyl-4′,4″-diaminodiphenoxy)-benzene, 1,4-bis(4′-aminophenoxy)-2-[(3′,5′-ditrifluoromethyl)phenyl]benzene, 1,4-bis[2′-cyano-3′(“4-amino phenoxy)phenoxy]-2-[(3′,5′-ditrifluoro-methyl)phenyl]benzene (6FC-diamine), 3,5-diamino-4-methyl-2′,3′,5′,6′-tetrafluoro-4′-tri-fluoromethyldiphenyloxide, 2,2-Bis[4′(4″-aminophenoxy)phenyl]phthalein-3′,5′-bis(trifluoromethyl)anilide (6FADAP) and 3,3′,5,5′-tetrafluoro-4,4′-diamino-diphenylmethane (TFDAM). In a specific embodiment, the fluorinated diamine is 2,2′-bis(trifluoromethyl) benzidine (TFMB).

In one embodiment, any number of additional diamines can be used in forming the polyimide film, including p-phenylenediamine (PPD), m-phenylenediamine (MPD), 2,5-dimethyl-1,4-diaminobenzene, 2,5-dimethyl-1,4-phenylenediamine (DPX), 2,2-bis-(4-aminophenyl) propane, 1,4-naphthalenediamine, 1,5-naphthalenediamine, 4,4′-diaminobiphenyl, 4,4″-diaminoterphenyl, 4,4′-diamino benzanilide, 4,4′-diaminophenyl benzoate, 4,4′-diaminobenzophenone, 4,4′-diaminodiphenylmethane (MDA), 4,4′-diaminodiphenyl sulfide, 4,4′-diaminodiphenyl sulfone, 3,3′-diaminodiphenyl sulfone, bis-(4-(4-aminophenoxy)phenyl sulfone (BAPS), 4,4′-bis-(aminophenoxy)biphenyl (BAPB), 4,4′-diaminodiphenyl ether (ODA), 3,4′-diaminodiphenyl ether, 4,4′-diaminobenzophenone, 4,4′-isopropylidenedianiline, 2,2′-bis-(3-aminophenyl)propane, N,N-bis-(4-aminophenyl)-n-butylamine, N,N-bis-(4-aminophenyl) methylamine, 1,5-diaminonaphthalene, 3,3′-dimethyl-4,4′-diaminobiphenyl, m-amino benzoyl-p-amino anilide, 4-aminophenyl-3-aminobenzoate, N,N-bis-(4-aminophenyl) aniline, 2,4-diaminotoluene, 2,5-diaminotoluene, 2,6-diaminotoluene, 2,4-diamine-5-chlorotoluene, 2,4-diamine-6-chlorotoluene, 2,4-bis-(beta-amino-t-butyl) toluene, bis-(p-beta-amino-t-butyl phenyl) ether, p-bis-2-(2-methyl-4-aminopentyl) benzene, m-xylylene diamine, and p-xylylene diamine.

Other useful diamines include 1,2-bis-(4-aminophenoxy)benzene, 1,3-bis-(4-aminophenoxy) benzene, 1,2-bis-(3-aminophenoxy)benzene, 1,3-bis-(3-aminophenoxy) benzene, 1-(4-aminophenoxy)-3-(3-aminophenoxy) benzene, 1,4-bis-(4-aminophenoxy) benzene, 1,4-bis-(3-aminophenoxy) benzene, 1-(4-aminophenoxy)-4-(3-aminophenoxy) benzene, 2,2-bis-(4-[4-aminophenoxy]phenyl) propane (BAPP), 2,2′-bis-(4-phenoxy aniline) isopropylidene, 2,4,6-trimethyl-1,3-diaminobenzene and 2,4,6-trimethyl-1,3-diaminobenzene.

Dianhydrides

In one embodiment, any number of suitable dianhydrides can be used in forming the polyimide film. The dianhydrides can be used in their tetra-acid form (or as mono, di, tri, or tetra esters of the tetra acid), or as their diester acid halides (chlorides). However, in some embodiments, the dianhydride form can be preferred, because it is generally more reactive than the acid or the ester.

Examples of suitable dianhydrides include 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 1,2,5,6-naphthalene tetracarboxylic dianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride, 2,3,6,7-naphthalene tetracarboxylic dianhydride, 2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzimidazole dianhydride, 2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzoxazole dianhydride, 2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzothiazole dianhydride, 2,2′,3,3′-benzophenone tetracarboxylic dianhydride, 2,3,3′,4′-benzophenone tetracarboxylic dianhydride, 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 2,2′,3,3′-biphenyltetracarboxylic dianhydride, 2,3,3′,4′-biphenyltetracarboxylic dianhydride, bicyclo-[2,2,2]-octen-(7)-2,3,5,6-tetracarboxylic-2,3,5,6-dianhydride, 4,4′-thio-diphthalic anhydride, bis (3,4-dicarboxyphenyl) sulfone dianhydride, bis (3,4-dicarboxyphenyl) sulfoxide dianhydride (DSDA), bis (3,4-dicarboxyphenyl oxadiazole-1,3,4) p-phenylene dianhydride, bis (3,4-dicarboxyphenyl) 2,5-oxadiazole 1,3,4-dianhydride, bis 2,5-(3′,4′-dicarboxydiphenylether) 1,3,4-oxadiazole dianhydride, 4,4′-oxydiphthalic anhydride (ODPA), bis (3,4-dicarboxyphenyl) thio ether dianhydride, bisphenol A dianhydride (BPADA), bisphenol S dianhydride, bis-1,3-isobenzofurandione, 1,4-bis(4,4′-oxyphthalic anhydride) benzene, bis (3,4-dicarboxyphenyl) methane dianhydride, cyclopentadienyl tetracarboxylic dianhydride, ethylene tetracarboxylic dianhydride, perylene 3,4,9,10-tetracarboxylic dianhydride, pyromellitic dianhydride (PMDA), tetrahydrofuran tetracarboxylic dianhydride, 1,3-bis-(4,4′-oxydiphthalic anhydride) benzene, 2,2-bis(3,4-dicarboxyphenyl) propane dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, phenanthrene-1,8,9,10-tetracarboxylic dianhydride, pyrazine-2,3,5,6-tetracarboxylic dianhydride, benzene-1,2,3,4-tetracarboxylic dianhydride and thiophene-2,3,4,5-tetracarboxylic dianhydride.

In one embodiment, a suitable dianhydride can include an alicyclic dianhydride, such as cyclobutane-1,2,3,4-tetracarboxylic diandydride (CBDA), 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA), hexahydro-4,8-ethano-1H,3H-benzo[1,2-c:4,5-c′]difuran-1,3,5,7-tetrone (BODA), 3-(carboxymethyl)-1,2,4-cyclopentanetricarboxylic 1,4:2,3-dianhydride (TCA), and meso-butane-1,2,3,4-tetracarboxylic dianhydride. In one embodiment, an alicyclic dianhydride can be present in an amount of about 70 mole percent or less, based on the total dianhydride content of the polyimide.

In one embodiment, a suitable dianhydride for forming the polyimide film can include a fluorinated dianhydride, such as 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 9,9-bis (trifuoromethyl)-2,3,6,7-xanthene tetracarboxylic dianhydride. In a specific embodiment, the fluorinated dianhydride is 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA).

Polyimide Films

In one embodiment, a polyimide film can be produced by combining a diamine and a dianhydride (monomer or other polyimide precursor form) together with a solvent to form a polyamic acid (also called a polyamide acid) solution. The dianhydride and diamine can be combined in a molar ratio of about 0.90 to 1.10. The molecular weight of the polyamic acid formed therefrom can be adjusted by adjusting the molar ratio of the dianhydride and diamine.

Useful methods for producing polyamic acid solutions in accordance with the present invention can be found in U.S. Pat. Nos. 5,166,308 and 5,298,331, which are incorporate by reference into this specification for all teachings therein. Numerous variations are also possible, such as,

(a) A method wherein the diamine components and dianhydride components are preliminarily mixed together and then the mixture is added in portions to a solvent while stirring.

(b) A method wherein a solvent is added to a stirring mixture of diamine and dianhydride components. (contrary to (a) above)

(c) A method wherein diamines are exclusively dissolved in a solvent and then dianhydrides are added thereto at such a ratio as allowing to control the reaction rate.

(d) A method wherein the dianhydride components are exclusively dissolved in a solvent and then amine components are added thereto at such a ratio to allow control of the reaction rate.

(e) A method wherein the diamine components and the dianhydride components are separately dissolved in solvents and then these solutions are mixed in a reactor.

(f) A method wherein the polyamic acid with excessive amine component and another polyamic acid with excessive dianhydride component are preliminarily formed and then reacted with each other in a reactor, particularly in such a way as to create a non-random or block copolymer.

(g) A method wherein a specific portion of the amine components and the dianhydride components are first reacted and then the residual diamine components are reacted, or vice versa.

(h) A method wherein the components are added in part or in whole in any order to either part or whole of the solvent, also where part or all of any component can be added as a solution in part or all of the solvent.

(i) A method of first reacting one of the dianhydride components with one of the diamine components giving a first polyamic acid. Then reacting another dianhydride component with another amine component to give a second polyamic acid. Then combining the amic acids in any one of a number of ways prior to imidization.

In one embodiment, a polyamic acid solution can be combined with conversion chemicals like: (i) one or more dehydrating agents, such as, aliphatic acid anhydrides and/or aromatic acid anhydrides (acetic anhydride, propionic anhydride, n-butyric anhydride, benzoic anhydride, trifluoroacetic anhydride and others); and (ii) one or more catalysts, such as, aliphatic tertiary amines (triethyl amine, etc.), aromatic tertiary amines (dimethyl aniline, etc.) and heterocyclic tertiary amines (pyridine, alpha, beta and gamma picoline (2-methylpyridine, 3-methylpyridine, 4-methylpyridine), isoquinoline, etc.). The anhydride dehydrating material is often used in molar excess compared to the amount of amide acid groups in the polyamic acid. The amount of acetic anhydride used is typically about 2.0-4.0 moles per equivalent (repeat unit) of polyamic acid. Generally, a comparable amount of tertiary amine catalyst is used.

In one embodiment, a conversion chemical can be an imidization catalyst (sometimes called an “imidization accelerator”) that can help lower the imidization temperature and shorten the imidization time. Typical imidization catalysts can range from bases such as imidazole, 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole, 2-phenylimidazole, benzimidazole, isoquinoline, substituted pyridines such as methyl pyridines, lutidine, and trialkylamines and hydroxy acids such as isomers of hydroxybenzoic acid. The ratio of these catalysts and their concentration in the polyamic acid layer will influence imidization kinetics and the film properties.

In one embodiment, the polyamic acid solution can be heated, optionally in the presence of the imidization catalyst, to partially or fully imidize the polyamic acid, converting it to a polyimide. Temperature, time, and the concentration and choice of imidization catalyst can impact the degree of imidization of the polyamic acid solution. Preferably, the solution should be substantially imidized. In one embodiment, for a substantially polyimide solution, greater than 85%, greater than 90%, or greater than 95% of the amic acid groups are converted to the polyimide, as determined by infrared spectroscopy.

In one embodiment, the solvated mixture (the substantially imidized solution) can be cast to form a polyimide film. In another embodiment, the solvated mixture (the first substantially imidized solution) can be precipitated with an antisolvent, such as water or alcohols (e.g., methanol, ethanol, isopropyl alcohol), and the solid polyimide resin can be isolated. For instance, isolation can be achieved through filtration, decantation, centrifugation and decantation of the supernatant liquid, distillation or solvent removal in the vapor phase, or by other known methods for isolating a solid precipitate from a slurry. In one embodiment, the precipitate can be washed to remove the catalyst. After washing, the precipitate may be substantially dried, but need not be completely dry. The polyimide precipitate can be re-dissolved in a second solvent, such as methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK), ethyl acetate, methyl acetate, ethyl formate, methyl formate, tetrahydrofuran, acetone, DMAc, NMP and mixtures thereof, to form a second substantially imidized solution (a casting solution), which can be cast to form a polyimide film.

The casting solution can further comprise any one of a number of additives, such as processing aids (e.g., oligomers), antioxidants, light stabilizers, flame retardant additives, anti-static agents, heat stabilizers, ultraviolet absorbing agents, inorganic fillers or various reinforcing agents. Inorganic fillers can include thermally conductive fillers, metal oxides, inorganic nitrides and metal carbides, and electrically conductive fillers like metals. Common inorganic fillers are alumina, silica, diamond, clay, boron nitride, aluminum nitride, titanium dioxide, dicalcium phosphate, and fumed metal oxides. Low color organic fillers, such as polydialkylfluorenes, can also be used.

In one embodiment, the elastic modulus of a polyimide film can be increased by the presence of sub-micron fillers. The percent transmittance of the consolidated film will be a function of the refractive index difference between the filler and the polymer host and the size of the filler. Smaller differences in the refractive index between the filler and the polymer host will allow for larger dimensions of the filler without adversely affecting the transmittance of the film. The sub-micron filler can be inorganic or organic and can be present in an amount between and optionally including any two of the following percentages: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 and 60 weight percent of the polyimide film.

Depending on the refractive index of the filler, in one embodiment the sub-micron filler can have a size of less than 550 nm in at least one dimension. In other embodiments, the filler can have a size of less than 500, less than 450, less than 400, less than 350, less than 300, less than 250, less than 200 nm, or less than 100 nm in at least one dimension (since fillers can have a variety of shapes in any dimension and since filler shape can vary along any dimension, the “at least one dimension” is intended to be a numerical average along that dimension). The average aspect ratio of the filler can be 1, for spherical particles, or greater than 1 for non-spherical particles. In some embodiments, the sub-micron filler is selected from a group consisting of needle-like fillers (acicular), fibrous fillers, platelet fillers, polymer fibers, and mixtures thereof. In one embodiment, the sub-micron filler is substantially non-aggregated. The sub-micron filler can be hollow, porous, or solid, or can have a core-shell structure where one composition is in the core and a second composition is in the shell. In one embodiment, the sub-micron fillers of the present disclosure exhibit an aspect ratio of at least 1:1, at least 2:1, at least 4:1, at least 6:1, at least 8:1, at least 10:1, at least 12:1, or at least 15:1.

In some embodiments, sub-micron fillers are 100 nm in size or less in at least one dimension. In some embodiments, the fillers are spherical or oblong in shape and are nanoparticles. In one embodiment, sub-micron fillers can include inorganic oxides, such as oxides of silicon, aluminum and titanium, hollow (porous) silicon oxide, antimony oxide, zirconium oxide, indium tin oxide, antimony tin oxide, mixed titanium/tin/zirconium oxides, and binary, ternary, quaternary and higher order composite oxides of one or more cations selected from silicon, titanium, aluminum, antimony, zirconium, indium, tin, zinc, niobium and tantalum. In one embodiment, nanoparticle composites (e.g. single or multiple core/shell structures) can be used, in which one oxide encapsulates another oxide in one particle.

In one embodiment, sub-micron fillers can include other ceramic compounds, such as boron nitride, aluminum nitride, ternary or higher order compounds containing boron, aluminum and nitrogen, gallium nitride, silicon nitride, aluminum nitride, zinc selenide, zinc sulfide, zinc telluride, and their combinations, or higher order compounds containing multiple cations and multiple anions.

In one embodiment, solid silicon oxide nanoparticles can be produced from sols of silicon oxides (e.g., colloidal dispersions of solid silicon oxide nanoparticles in liquid media), especially sols of amorphous, semi-crystalline, and/or crystalline silica. Such sols can be prepared by a variety of techniques and in a variety of forms, which include hydrosols (i.e., where water serves as the liquid medium), organosols (i.e., where organic liquids serves as the liquid medium), and mixed sols (i.e., where the liquid medium comprises both water and an organic liquid). See, e.g., descriptions of the techniques and forms disclosed in U.S. Pat. Nos. 2,801,185, 4,522,958 and 5,648,407. In one embodiment, the nanoparticle is suspended in a polar, aprotic solvent, such as, DMAc or other solvent compatible with polyamic acid or polyimide solution In another embodiment, solid silicon oxide nanoparticles can be commercially obtained as colloidal dispersions or sols dispersed in polar aprotic solvents, such as for example DMAC-ST (Nissan Chemical America Corporation, Houston Tex.), a solid silica colloid in dimethylacetamide containing less than 1 percent water by weight, with 20-21 wt % SiO₂, with a median nanosilica particle diameter d50 of about 20 nm.

In one embodiment, sub-micron fillers can be porous and can have pores of any shape. One example is where the pore comprises a void of lower density and low refractive index (e.g., a void containing air) formed within a shell of an oxide such as silicon oxide, i.e., a hollow silicon oxide nanoparticle. The thickness of the sub-micron fillers shell affects the strength of the sub-micron fillers. As the hollow silicon oxide particle is rendered to have reduced refractive index and increased porosity, the thickness of the shell decreases resulting in a decrease in the strength (i.e., fracture resistance) of the sub-micron fillers. Methods for producing such hollow silicon oxide nanoparticles are known, for example, as described in Japanese Patent Nos. 4406921B2 and 403162462. Hollow silicon oxide nanoparticles can be obtained from JGC Catalysts and Chemicals, LTD, Japan.

In one embodiment, sub-micron fillers can be coated with a coupling agent. For example, a nanoparticle can be coated with an aminosilane, phenylsilane, acrylic or methacrylic coupling agents derived from the corresponding alkoxysilanes. Trimethylsilyl surface capping agents can be introduced to the nanoparticle surface by reaction of the sub-micron fillers with hexamethyldisilazane. In one embodiment, sub-micron fillers can be coated with a dispersant. In one embodiment, sub-micron fillers can be coated with a combination of a coupling agent and a dispersant. Alternatively, the coupling agent, dispersant or a combination thereof can be incorporated directly into the polyimide film and not necessarily coated onto the sub-micron fillers.

The surface coating on an inorganic sub-micron filler will affect its refractive index. The refractive index of sub-micron fillers with a surface coating can be estimated by summing the volume fraction of the surface coating multiplied by its refractive index and the volume fraction of the inorganic core multiplied by the refractive index of the core. (this can't hurt but is optional)

In some embodiments, the sub-micron filler is chosen so that it does not itself degrade or produce off-gasses at the desired processing temperatures. Likewise, in some embodiments, the sub-micron filler is chosen so that it does not contribute to degradation of the polymer.

In one embodiment, the substantially imidized polyimide solution can be cast or applied onto a support, such as an endless belt or rotating drum, to form a film. Alternatively, it can be cast on a polymeric carrier such as PET, other forms of Kapton® polyimide film (e.g., Kapton® HN or Kapton® OL films) or other polymeric carriers. Next, the solvent containing-film can be converted into a film by heating to partially or fully remove the solvent. In some aspects of the invention, the film is separated from the carrier before drying to completion. Final drying steps can be performed with dimensional support or stabilization of the film. In other aspects, the film is heated directly on the carrier.

The thickness of the polyimide film may be adjusted, depending on the intended purpose of the film or final application specifications. In one embodiment, the polyimide film has a total thickness in a range of from about 10 to about 150 μm, or from about 10 to about 100 μm, or from about 25 to about 80 μm.

In one embodiment, the polyimide film has a tensile modulus of at least about 5.5 GPa or at least about 6.0 GPa or at least about 6.5 GPa.

In one embodiment, the polyimide film has a b* of less than about 1.4, or less than about 1.25, or less than about 1.0 or less than about 0.8 for a film thickness of about 50 μm, when measured with a dual-beam spectrophotometer, using D65 illumination and 10 degree observer, in total transmission mode over a wavelength range of 360 to 780 nm. In one embodiment, the polyimide film has a yellowness index (YI) of less than about 2.25, or less than about 2.0 or less than about 1.75 for a film thickness of about 50 μm, when measured using the procedure described by ASTM E313.

Applications

In one embodiment, a polyimide film with low color and high tensile strength can be used for a number of layers in electronic device applications, such as in an organic electronic device, where a combination of good optical and mechanical properties is desirable. Nonlimiting examples of such layers include device substrates, touch panels, substrates for color filter sheets, cover films, and others. The particular materials' properties requirements for each application are unique and may be addressed by appropriate composition(s) and processing condition(s) for the polyimide films disclosed herein. Organic electronic devices that may benefit from having a coated film include, but are not limited to, (1) devices that convert electrical energy into radiation (e.g., a light-emitting diode, light emitting diode display, lighting device, luminaire, or diode laser), (2) devices that detect signals through electronics processes (e.g., photodetectors, photoconductive cells, photoresistors, photoswitches, phototransistors, phototubes, IR detectors, biosensors), (3) devices that convert radiation into electrical energy, (e.g., a photovoltaic device or solar cell), (4) devices that convert light of one wavelength to light of a longer wavelength, (e.g., a down-converting phosphor device); and (5) devices that include one or more electronic components that include one or more organic semi-conductor layers (e.g., a transistor or diode).

The advantageous properties of this invention can be observed by reference to the following examples that illustrate, but do not limit, the invention. All parts and percentages are by weight unless otherwise indicated.

EXAMPLES Test Methods Measurement of CIE L*, a*, b* Color and Yellowness Index

Color measurements were performed using a ColorQuest® XE dual-beam spectrophotometer (Hunter Associates Laboratory, Inc., Reston, Va.), using D65 illumination and 10 degree observer, in total transmission mode over a wavelength range of 360 to 780 nm. Yellowness Index (YI) was measured using the procedure described by ASTM E313.

Transmittance and Haze

Transmittance and haze were measured using a Haze-Guard Plus (BYK-Gardner GmbH, Germany), with the haze measured in transmission by collecting forward scattered light using the method described by ASTM1003. Percent haze was determined by measuring the amount of light which deviates from the incident beam by more than 2.5 degrees on average.

Tensile Modulus

Tensile modulus was measured using the ASTM D882 test method.

Percent Imidization

A polyimide film was cast from solution and dried at 25° C. (10 millitorr) for 16 hours. Attenuated Total Reflectance Fourier Transform Infra-Red (ATR-FTIR) spectroscopy measurements were performed with a single bounce germanium ATR accessory, installed in a FTIR spectrometer (Nicolet™ iS50, Thermo Fisher Scientific, Inc., Waltham, Mass.). The ratio of intensities at 1365 cm⁻¹ (polyimide C-N) relative to 1492 cm⁻¹ (aromatic stretch used as an internal standard) was used to characterize cure, relative to a sample prepared with standard curing methods that was defined as being 100% cured. Both sides of the film were measured to determine the overall percent imidization.

Thickness

Coating thickness was determined by measuring coated and uncoated samples in 5 positions across the profile of the film using a contact-type FISCHERSCOPE MMS PC2 modular measurement system thickness gauge (Fisher Technology Inc., Windsor, Conn.).

Comparative Examples 1 and 2

For the polyamic acid (PAA) solution of Comparative Examples 1 and 2 (CE1 and CE2) with a monomer composition of CBDA 0.4/6FDA 0.6//TFMB 1.0, into a 500-ml nitrogen purged resin kettle, 61.2547 g of trifluoromethyl-benzidine (TFMB, Seika Corp., Wakayama Seika Kogyo Co., LTD., Japan) was added along with 381.96 g of dimethyl acetamide (DMAc, HPLC grade). 50.5138 g of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA, Synasia Inc., Metuchen, N.J.) and 14.7049 g of cyclobutane 1,2,3,4-tetracarboxylic dianhydride (CBDA, Wilshire Technologies, Princeton, N.J.) were added in three aliquots over three 5-10 minute intervals. An additional 94.49 g of DMAc was added. The reaction mixture was held at 40° C. during these additions.

The polymer was polymerized (“finished”) to ˜975 poise (weight average molecular weight, M_(w)=290,480 Daltons, PDI 2.17) using small additions of 6 wt % 6FDA solution in DMAc.

The polyamic acid was de-gassed using a centrifugal-planetary mixer (THINKY USA, Laguna Hills, Calif.) to force the gas from the pre-polymer at 2000 rpm for 2 minutes followed by 2200 rpm for 2 minutes. This procedure was repeated if further de-gassing of the polymer was needed.

47.00 g of this polyamic acid solution in DMAc was placed in a freezer and cooled to ˜−5° C. 3.48 g of b-picoline (Sigma-Aldrich, Milwaukee, Wis.) and 3.81 g of acetic anhydride (Sigma-Aldrich) were combined with the polyamic acid mixture at ˜−5° C. The polyamic acid mixture with beta-picoline and acetic anhydride was maintained at −5 to −10° C. to minimize imidization of the solution. It was mixed and de-gassed using the centrifugal-planetary mixer to force the gas from the pre-polymer at 2000 rpm for 1 minute followed by 2200 rpm for 30 seconds.

The solution was cast onto a glass plate at 25° C. using a doctor blade with a 25-mil clearance to produce ˜2 mil films after curing. The film on the glass substrate was heated to 80° C. for 30 minutes and was subsequently lifted off the glass surface and mounted onto a 4×8 inch pin frame. The mounted film was placed in a furnace (Thermolyne™ F6000 box furnace, Thermo Fisher Scientific, Inc., Waltham, Mass.). The furnace was purged with nitrogen and heated according to the following temperature protocol:

25 to 45° C. (5° C./min), hold at 45° C. for 5 minutes;

45 to 150° C. (20° C./min), hold at 150° C. for 10 minutes;

150 to 250° C. (20° C./min), hold at 250° C. for 10 minutes;

250 to 300° C. (20° C./min), hold at 300° C. for 5 minutes.

The films were removed “hot” from the oven after heating to 300° C. for 5 minutes and allowed to cool in air.

Comparative Example 3

For Comparative Example 3 (CE3), to prepare a substantially imidized polyimide solution (polyimide amic acid solution), 60.83 g of the PAA solution from CE1/CE2 was added to a 500-ml nitrogen purged resin kettle. 4.50 g of beta-picoline and 4.93 g of acetic anhydride were combined with the PAA solution. An additional 6.21 g of DMAc was added. The reaction mixture was stirred unheated for 30 minutes then heated to 80° C. for 1 hour to imidize the solution. 43.0 g of cooled polymer solution was poured into 100 g of rapidly stirring methanol in a blender. The pulverized polymer solid was allowed to stir in the blender for 10 minutes before collection by filtration. The polymer was dried under vacuum at 25° C. for ˜16 hours. The dried polymer was added to 43.19 g of DMAc and mixed in the centrifugal-planetary mixer to obtain a solution. Infrared data shows that the polymer is substantially converted (96.1%) to the polyimide form.

The solution was de-gassed using the centrifugal-planetary mixer to force the gas from the polymer at 2000 rpm for 2 minutes followed by 2200 rpm for 2 minutes. This procedure was repeated if further de-gassing of the polymer was needed.

The solution was cast onto a glass plate at 25° C. using a doctor blade with a 20-mil clearance to produce ˜2 mil cured films. The film on the glass substrate was heated to 80° C. for 30 minutes and was subsequently lifted off the glass surface and mounted onto a 4×8 inch pin frame. The mounted film was placed in a furnace. The furnace was purged with nitrogen and heated following the same temperature protocol as described above for CE1/CE2. The film was removed “hot” from the oven after heating to 300° C. for 5 minutes and allowed to cool in air.

Comparative Example 4

For Comparative Example 4 (CE4), the same procedure as described for CE3 was used to form the solution and prepare the film, but a different heating profile was used, stopping at a lower temperature for the final cure. The mounted film was placed in the furnace, which was then purged with nitrogen and heated according to the following temperature protocol:

25 to 45° C. (5° C./min), hold at 45° C. for 5 minutes;

45 to 150° C. (20° C./min), hold at 150° C. for 10 minutes;

150 to 250° C. (20° C./min), hold at 250° C. for 15 minutes; The film was removed “hot” from the oven after heating to 250° C. for 15 minutes and allowed to cool in air.

Example 1

For Example 1 (E1), the same procedure as described for CE3 for the preparation of polyamic acid and polyimide resin was used with the following differences. 66.713 g of TFMB was added along with 479.66 g of DMAc, 36.571 g of 6FDA and 24.219 g of CBDA to prepare a polyamic acid solution with a monomer composition of CBDA 0.6/6FDA 0.4//TFMB 1.0.

The polymer was “finished” to 1475 poise (weight average molecular weight, M_(w)=363,739 Daltons, PDI 2.18) using small additions of 6 wt % 6FDA solution in DMAc.

As described in CE3, a portion of the polyamic acid was converted to polyimide, the polymer isolated by precipitation and the dried resin dissolved and cast into films with the following differences. To 103.50 g of polyamic acid solution in a 500-ml nitrogen purged resin kettle, 8.268 g of beta-picoline and 9.064 g of acetic anhydride were added. 11.75 g of the polyimide resin was combined with 45.0 g of DMAc and mixed to obtain a solution and cast as a film.

The mounted film was placed in a furnace. The furnace was purged with nitrogen and heated according to the following temperature protocol:

25 to 45° C. (5° C./min), hold at 45° C. for 5 minutes;

45 to 150° C. (20° C./min), hold at 150° C. for 10 minutes;

150 to 250° C. (20° C./min), hold at 250° C. for 15 minutes.

The films were removed “hot” from the oven after heating to 250° C. for 15 minutes and allowed to cool in air.

Example 2

For Example 2 (E2), with a monomer composition of BPDA 0.1/CBDA 0.6/6FDA 0.3//TFMB 1.0, to a 1000-ml liter nitrogen purged resin kettle, 35.00 g of TFMB was added along with 422.5 g of DMAc. The reactor was heated to 40° C. and held at this temperature for all subsequent reagent additions. 14.391 g of 6FDA, 12.706 g of CBDA and 3.177 g 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA, Mitsubishi Chemical Co., Japan) were gently blended in an inert atmosphere until the combined powder was visually uniform. The blended powder was added in three equal portions to the solution containing TFMB and DMAc. The solution was allowed to react with the dianhydrides for 15-20 minutes after each addition. After the addition of the dianhydrides was complete, 105.6 g of DMAc was added to the solution. The polymer was polymerized or chain extended at 40° C. to a measured viscosity of ˜20 poise using small additions of 10 wt % 6FDA solution in DMAc.

To form the partially or fully imidized polyimide solution (polyimide amic acid solution), a catalyst solution was prepared by mixing 26.12 g of beta-picoline and 28.63 g of acetic anhydride until uniform. A dropping funnel with nitrogen purge was added to the reactor described in the previous section, and the polyamic acid solution was heated to 40° C. The catalyst solution was added to 888.12 g of the polyamic acid solution using the dropping funnel. The catalyst solution was added slowly over the course of 20-30 min. The reaction mixture was stirred at 40° C. for 30 minutes and then heated to 80° C. for 2 hours to complete the reaction procedure.

620 g of the partially or fully imidized solution was precipitated with methanol. 500 ml methanol was added to a 1-L stainless steel blender along with 180 g of the partially or fully imidized solution and blended for 3 minutes, alternating between low and high speed. The mixture was allowed to settle for 5 minutes and the liquid decanted into a funnel. The solid precipitate (remaining in the blender) was washed by adding 200 ml of methanol and blending for 2 minutes. The polymer precipitate was allowed to settle for 5 minutes and the methanol was decanted into the funnel. This washing process was repeated two additional times using another portion (220 g) of the partially or fully imidized solution. The decanted methanol and additional precipitated resin were added to the same funnel containing the precipitated resin. The polymer precipitate was dried under vacuum for approximately 12 hours on the filter funnel. The precipitated solids were transferred into a vacuum drying oven and dried under vacuum for approximately 24 hours at 60° C.

To prepare a 10% polymer resin solution, 6 g of the polymer resin and 54 g of DMAc were combined. The solution was mixed in the centrifugal-planetary mixer at 2000 rpm for 8 minutes. The process was then repeated for a second 8 minute cycle. The solution was placed on a roller and allow to turn for ˜16 hours. The solution was filtered through a 5 μm filter. The final viscosity of the solution was 157 poise at 25° C.

To form the imidized film, the solution was de-gassed using the centrifugal-planetary mixer to force the gas from the polymer at 2000 rpm for 2 minutes followed by 2200 rpm for 2 minutes. This procedure was repeated if further de-gassing of the polymer was needed. The solution was cast on a glass plate at 25° C. using a doctor blade with a 40-mil clearance to produce ˜2 mil dried films. The film on the glass substrate was heated to 50° C. for 30 minutes followed by 90° C. for 30 minutes and was subsequently lifted off the glass surface and mounted onto a 4×8 inch pin frame.

The mounted film was placed in a furnace. The furnace was purged with nitrogen and heated according to the following temperature protocol:

90° C. for 5 minutes;

90 to 150° C. (7° C./min), hold at 150° C. for 10 minutes;

150 to 250° C. (7° C./min), hold at 250° C. for 20 minutes.

The films were removed “hot” from the oven after heating to 250° C. for 15 minutes and allowed to cool in air.

Examples 3 and 4

For Examples 3 and 4 (E3 and E4), the same procedure as described for CE3 for the preparation of polyamic acid and polyimide solution was used with the following differences. 47.044 g of TFMB was added along with 554.46 g of DMAc, 26 g of 6FDA and 17.217 g of CBDA to prepare a polyamic acid solution with a monomer composition of CBDA 0.6/6FDA 0.4//TFMB 1.0. The polymer reached a weight average molecular weight, M_(w)=237,468 Daltons, PDI 1.59.

As described in CE3, the polyamic acid was converted to polyimide, the polymer isolated by precipitation and the dried resin dissolved and cast into films with the following differences. To convert the polyamic acid solution in a 1-L liter nitrogen purged resin kettle into a polyimide solution, 24.98 g of pyridine, 32.24 g of acetic anhydride and DMAc were added to end with a 9-10 wt % solution.

To prepare a sub-micron filler, an aqueous silica colloid (Ludox® TMA, slightly acidic form of colloidal silica, 34 wt % in water, (ph˜7.0), W.R. Grace and Co., Columbia Md.) was used. 762 kg of this colloid was combined with 701 kg of DMAc (99.8%) in a 500-gallon distillation reactor. Water was distilled from the colloid until a water level of 0.5-1 wt % was achieved (distillation temperature ˜100° C.). 25.4 kg of phenyltrimethoxysilane (Dow Corning, Midland, Mich.) was added to this reaction mixture, and the reactor was further heated (temp ˜100° C.) until the water level was <800 ppm. The final colloid concentration was ˜31.44 wt %.

For E3, 6 g of the polyimide resin was combined with 50.72 g of DMAc and 4.78 g of 31.40 wt % silica colloid particles in DMAc and mixed to obtain a filled solution. The solution was de-gassed using the centrifugal-planetary mixer to force the gas from the polymer at 2000 rpm for 10 minutes.

The solution was cast onto a glass substrate at 25° C. to produce 1-2 mil cured films. The film on the glass substrate was heated to 80° C. for 25 minutes, allowed to cool, and subsequently lifted off the glass surface and mounted onto an 8×12 inch frame. The mounted film was placed in a furnace. The furnace was heated from 45 to 220° C. (16° C./min), then held at 220° C. for 15 minutes. The film was removed “hot” from the oven after heating to 220° C. for 15 minutes and allowed to cool in air. The dried film contained 20 wt % of sub-micron filler.

For the filled solution of E4, 6 g of the polyimide resin was combined with 48.39 g of DMAc and 8.18 g of 31.40 wt % silica colloid particles in DMAc and mixed to obtain a filled solution. The solution was de-gassed, cast onto a glass substrate and heated as described for E3. The dried film contained 30 wt % of sub-micron filler.

TABLE 1 Thickness YI Haze Modulus Example (μm) a* b* L* (E313) (%) (GPa) CE1 55.1 −0.21 1.32 96.20 2.35 0.15 4.3 CE2 55.6 −0.25 1.54 96.17 2.74 0.08 4.3 CE3 49.8 −0.08 0.89 96.15 1.62 0.12 4.5 CE4 47.8 −0.04 0.71 96.30 1.32 0.07 4.3 E1 47.0 −0.03 0.94 96.0 1.77 0.34 5.8 E2 47.8 −0.12 1.01 95.93 1.84 — 6.8 E3 31.2 −0.33 1.01 95.27 1.66 0.96 5.6 E4 31.0 −0.35 1.04 95.40 1.70 0.96 6.4

As shown in Table 1, the desirable properties of low color and high modulus can be achieved when a polymer which contains an aliphatic monomer is used in a process where the polyamic acid solution is substantially imidized in solution prior to casting and drying the film, for film both with (E1-E2) and without (E3-E4) sub-micron fillers.

Further benefits can be achieved when the catalyst is removed from the soluble polyimide solution, typically through precipitation of the polyimide resin, washing of the resin and re-dissolution of the resin to create the castable solution. More conventional methods, such as casting the polyamic acid solution to form films that are subsequently imidized, result in films having a higher color. This is shown in comparing CE1/CE2 with CE3. The polyamic acid solutions of CE1 and CE2 were cast with the catalysts present in the films as cast. CE3 is the same composition, but a washed and dried polyimide resin is formed and then put back into a solution for casting. The cast polyimide film was then heated using the same temperature profile to form the final film. The optical properties of CE3 are clearly improved compared to CE1/CE2, although the modulus is still somewhat low.

Note that not all of the activities described above in the general description are required, that a portion of a specific activity may not be required, and that further activities may be performed in addition to those described. Still further, the order in which each of the activities are listed are not necessarily the order in which they are performed. After reading this specification, skilled artisans will be capable of determining what activities can be used for their specific needs or desires.

In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. All features disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense and all such modifications are intended to be included within the scope of the invention.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. 

What is claimed is:
 1. A polyimide film comprising: a dianhydride and a diamine, wherein: the dianhydride, the diamine or both the dianhydride and the diamine comprise an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer; and the polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm.
 2. The polyimide film of claim 1, further comprising a sub-micron filler.
 3. The polyimide film of claim 2, wherein the sub-micron filler is selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, and mixtures thereof.
 4. The polyimide film of claim 2, wherein the sub-micron filler has a size of less than 100 nm in at least one dimension.
 5. The polyimide film of claim 1, wherein the dianhydride is selected form the group consisting of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride and mixtures thereof.
 6. The polyimide film of claim 5, wherein the dianhydride further comprises an alicyclic dianhydride selected from the group consisting of cyclobutane-1,2,3,4-tetracarboxylic dianhydride, cyclohexane dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, hexahydro-4,8-ethano-1H,3H-benzo[1,2-c:4,5-c′]difuran-1,3,5,7-tetrone, 3-(carboxymethyl)-1,2,4-cyclopentanetricarboxylic 1,4:2,3-dianhydride and meso-butane-1,2,3,4-tetracarboxylic dianhydride.
 7. The polyimide film of claim 1, wherein the diamine comprises a fluorinated aromatic diamine.
 8. The polyimide film of claim 7, wherein the fluorinated aromatic diamine comprises 2,2′-bis(trifluoromethyl) benzidine.
 9. The polyimide film of claim 1, wherein the diamine comprises an aliphatic diamine selected from the group consisting of 1,2-diaminoethane, 1,6-diaminohexane, 1,4-diaminobutane, 1,5 diaminopentane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,9-diaminononane, 1,10-diaminodecane, 1,11-diaminoundecane, 1,12-diaminododecane, 1,16-hexadecamethylenediamine, 1,3-bis(3-aminopropyl)-tetramethyldisiloxane, isophoronediamine, bicyclo[2.2.2]octane-1,4-diamine and mixtures thereof.
 10. The polyimide film of claim 1, wherein the diamine comprises an alicyclic diamine selected from the group consisting of cis-1,3-diaminocyclobutane trans-1,3-diaminocyclobutane, 6-amino-3-azaspiro[3.3]heptane, and 3,6-diaminospiro[3.3]heptane, bicyclo[2.2.1]heptane-1,4-diamine, isophoronediamine, bicyclo[2.2.2]octane-1,4 diamine, cis-1,4 cyclohexane diamine, trans-1,4 cyclohexane diamine, 1,4-bis(aminomethyl)cyclohexane, 4,4′-methylenebis(cyclohexylamine), 4,4′-methylenebis(2-methyl-cyclohexylamine), bis(aminomethyl)norbornane and mixtures thereof.
 11. The polyimide film of claim 1, wherein the polyimide film has a thickness in the range of from 10 to 150 μm.
 12. The polyimide film of claim 1, wherein the polyimide film has a L* of at least
 90. 13. The polyimide film of claim 1, wherein the polyimide film has a haze of less than 1%.
 14. An electronic device comprising the polyimide film of claim
 1. 15. The electronic device of claim 14, wherein the polyimide film is used in device components selected from the group consisting of substrates for color filter sheets, cover sheets, and touch sensor panels.
 16. A polyimide film comprising: a dianhydride and a diamine, wherein: the dianhydride, the diamine or both the dianhydride and the diamine comprise an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer; and the polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm; and the polyimide film is formed by: (a) polymerizing the dianhydride and the diamine in the presence of a first solvent to obtain a polyamic acid solution; (b) imidizing the polyamic acid solution to form a substantially imidized solution; (c) casting the substantially imidized solution to form a film; and (d) drying the film.
 17. The polyimide film of claim 16, wherein after (b) and before (c), the substantially imidized solution is filtered to remove insoluble constituents of the solution.
 18. A polyimide film comprising: a dianhydride and a diamine, wherein: the dianhydride, the diamine or both the dianhydride and the diamine comprise an alicyclic monomer, an aliphatic monomer or both an alicyclic monomer and an aliphatic monomer; and the polyimide film has a tensile modulus of 5.5 GPa or more, a b* of 1.4 or less and a yellowness index of 2.25 or less for a film thickness of 50 μm; and the polyimide film is formed by: (a) polymerizing the dianhydride and the diamine in the presence of a first solvent to obtain a polyamic acid solution; (b) imidizing the polyamic acid solution to form a first substantially imidized solution; (c) precipitating a solid polyimide resin from the first substantially imidized solution with an antisolvent; (d) isolating and drying the solid polyimide resin; (e) dissolving the solid polyimide resin in a second solvent to form a second substantially imidized solution; (f) casting the second substantially imidized solution to form a film; and (g) drying the film.
 19. The polyimide film of claim 18, wherein after (e) and before (f), the second substantially imidized solution is filtered to remove insoluble constituents of the solution.
 20. The polyimide film of claim 18, wherein the first and second solvents are the same or different. 